Metallurgical grain refinement process

ABSTRACT

A WIDELY APPLICABLE PROCESS FOR THE PRODUCTION OF METAL CASTINGS WITH A UNIFORM FINE GRAIN STRUCTURE IN THE AS-CAST CONDITION IS DISCLOSED. AN INOCULANT CONTAINING AT LEAST TWO CONSTITUENTS, ONE FOR LOCALIZED DEPRESSION OF THE ALLOY FREEZING POINT AND THE OTHER FOR SEEDING PURPOSES IS USED IN COMBINATION WITH RELATIVELY SLOW COOLING TO PROVIDE A CONTROLLED CONSTITUTIONAL SUPERCOOLING EFFECT DURING THE FREEZING OF THE ALLOY. RELATIVELY LARGE CASTINGS IN SAND MOLDS ARE POSSIBLE WITH A UNIFORM FINE GRAIN STRUCTURE THROUGHOUT.

United States Patent O 3,744,997 METALLURGICAL GRAHN REE'HNEMENT PROEESS Lawrence A. Deimen, Ann Arbor, Mich, assignor of a fractional part interest to James M. Dolmen, Picltney, Mich. No Drawing. Filed Oct. 6, 1969, Ser. No. 864,192 lnt. Cl. BZZd 27/16 US. Cl. '75l29 28 Claims ABTRAT OF THE DISCLOSURE A widely applicable process for the production of metal castings with a uniform fine grain structure in the as-cast condition is disclosed. An inoculant containing at least two constituents, one for localized depression of the alloy freezing point and the other for seeding purposes is used in combination with relatively slow cooling to provide a controlled constitutional supercooling effect during the freezing of the alioy. Relatively large castings in sand molds are possible with a uniform fine grain structure throughout.

BACKGROUND OF THE INVENTION Ordinarily when a molten metal is poured into a mold and solidifies, the solidification takes place from the surface of the casting section towards the center of the section since the surface of the mold is the coolest area. The common result is long narrow columnar grains that begin at the surface of the casting section and extend inwardly toward the center of the casting section. A metallurgically non-uniform grain structure occurs with the accompanying result of directional properties and relative weakness in the casting. It has been long known that a fine grain structure with a random orientation will result in a much strong casting with relatively uniform mechanical properties in all directions. Various techniques have been developed throughout the years for providing a fine grain structure in metals. Extensive heat treatment processes have been developed which are usually lengthy and must be specific for the particular alloy to be heat treated. Forging and hot rolling techniques have been developed for grain refinement where the final configuration of the product is not the same as the casting. Compared to the foregoing, casting itself is relatively inexpensive. Many configurations can only be made as a casting where an integral unit is desired. in addition to extra expense, further heat treatment of the casting may be undesirable due to dimensional changes.

Much research has been directed toward the develop ment of alloys, inoculants and casting techniques to provide a uniform fine grain structure in a casting without any additional steps beyond pouring and cooling the casting. Mechanical techniques include magnetic stirring and ultrasonic vibration. Magnetic stirring has been attempted with ferrous alloys and results in a fine grain structure throughout most of a simply shaped ingot. Problems arise however with complicated shapes and the need for expensive stirring equipment. The same is true for ultrasonic vibration techniques which require expensive additional equipment and may result in difficult problems when sand molds are used.

Quick freezing techniques have been attempted in order to limit the length of time that grains have to grow from the liquid. This technique however requires a high rate of heat transfer. In order to provide a high rate of heat transfer only relatively thin sections may be cast and permanent molds are generally required.

The greatest emphasis in recent years has been upon inoculation techniques. lnoculants have long been used for such purposes as deoxidation in the melt. Recently cer- Patented July 110, 1973 tain specific additives have been utilized as inoculants for fine grain structures in specific alloys. These inoculants however have been specific in the sense that they are limited in usefulness to certain alloys under relatively restricted conditions. An example of this is a calcium additive utilized for producing fine grain structures in certain steels. Specific inoculants have also been developed for the dispersion of carbides in hypereutectoid steels. A slightly different technique involves coating the surface of a mold with a compound which acts as an inoculant in refining the grain as the molten metal is poured into the mold. This technique is limited to relatively thin castings also and in addition requires the additional step in making the mold of coating the mold properly. The coating also is specific to the particular alloy to be cast. Another technique involves adding a finely divided inoculant that consists essentially of the specific base metal of an alloy just before pouring into the mold. A fine grain results because the additive does not completely melt and acts as nucleating bodies for the alloy as the alloy freezes shortly thereafter.

SUMMARY OF THE INVENTION The invention involves inoculating a molten metal just prior to pouring into a heated mold with an inoculant containing a combination of a broad range of ingredients. The same broad range of ingredients. The same broad spectrum inoculant may be used for both ferrous and non-ferrous metal alloys and pure metals in general with the limitation that the solidus temperature of the casting metal be substantially above the melting point of the lowest temperature melting point metal in the inoculant and the liquidus temperature be substantially below the melting point of the highest temperature melting point metal in the inoculant. After pouring into the mold, the mold must provide for slow cooling. The simplest method of providing slow cooling is to use a pre-heated mold as noted above and allow the mold and casting therein to air cool. Sand molds as well as metal molds are desirable. In general sand molds will provide a finer grain structure than permanent molds. The invention is particularly desirable for large sections in sand molds hitherto impossible due to the requirement for quick freezing in most other prior art processes. The amount of inoculant required is exceptionally low; usually less than 0.1% by weight.

DESCRIPTION OF THE PREP-ERRED EMBODIMENT In practising the invention the metal to be cast is prepared with a super heat temperature in the furnace of 45 F. to F. above the usual pouring temperature for proper fluidity. The inoculant is added to the metal in the furnace and pouring is begun int-o heated molds. The molds and castings are allowed to air cool. The process steps apply to both ferrous and nonferrous metal alloys and pure metals and both permanent molds and sand molds. An electric induction furnace has been found best because the superheat temperature may be controlled by simply turning the furnace on and off. Ladle addition is possible however so long as the metal temperature does not drop below the pouring temperature prior to pouring. The superheat temperature range above is preferred for greatest effectiveness but the method is not limited to this superheat range. Excessive superheat however may reduce the effectiveness of some of the seeding materials in the inoculant. The inoculation is most effective when done as the last step in the melting and refining procedure prior to pouring into the mold. Thus such procedures as deoxidation and degasification should be completed prior to inoculation. The inoculant is usually added by making a hole through the slag atop the molten metal and plunging in the inoculant contained in metal tubes attached to a plunger. The inoculant distributes itself throughout the melt in the few minutes time taken to position the metal for pouring. Inoculation prior to pouring is preferable to direct inoculation into the ingot or casting because sufiicient distribution may not be attained by post inoculation especially in the case of a complicated casting where vigorous motion of the molten metal is no longer occurring. The inoculant used must meet certain criteria. For a given casting metal the inoculant must contain metals with melting points substantially above the liquidus temperature of the metal being poured. The inoculant must also contain metals with melting points substantially below the solidus temperature of the metal being poured. An inoculant containing one high melting point metal as a seeding material and one low melting point metal as a freezing point depression element can be used. However, it has been found desirable to use a broad spectrum inoculant such as the composition shown in Table -I because such a broad spectrum inoculant may be used with a variety of metals, both ferrous and nonferrous. Only a small amount of the above inoculant is necessary. About 0.03 of a percent by weight of this particular inoculant is sufiicient to grain refine both ferrous and non-ferrous alloys.

TABLE I Percent Melting temperature of- Element composition Cu (copper) Fe (iron) Zn (zinc) Mn (manganese)- Cr (chromium) Ti (titanium) Ni (nickel) Rh (rhodium 5 HHNUIHI-ING I believe that the success of this particular process is based on the principle of constitutional supercooling. However, previous processes utilizing constitutional supercooling with an inoculant require quick freezing, thus limiting their usefulness. Under the principle of constitutional supercooling the freezing temperature of the alloy is slightly depressed in the region surrounding each grain as it grows by solidification from the molten alloy. The growth of the grain is slowed because of differing composition surrounding the grain with a lower freezing temperature. The change composition is due to the rejection as the alloy grain freezes of lower melting point metals added in the inoculant. Further away from the grain the molten alloy retains the original composition and original freezing temperature. However, the temperature gradient extending from the grain boundry gives rise to a supercooled region surrounding the grain. Within this supercooled region the high melting point seed material of the inoculant provides nucleating points for new grains to begin growing. The heated molds in this process cause a much shallower thermal gradient within the solidifying alloy. The relatively shallow thermal gradient of the present invention provides for a region around the grain slightly different from that described above. The

supercooled region is larger in volumetric spatial extent about the grain; however, the supercooled temperature differential between the actual temperature and the slightly higher freezing temperature is not as great. A greater number of nucleating sites is provided per unit time which explains the apparent paradox of the present invention with respect to the quick freezing requirement of the prior art. Slow cooling here promotes a fine grain structure Whereas general metallurgical considerations specify a fast cooling rate to attain a fine grain structure. The slow cooling technique of the present invention makes possible the large castings with a uniform grain structure throughout and the use of sand molds. Use of the present invention with a change to unheated molds results in a faster cooling rate for the ingot and a grain structure that is equiaxed but approximately five times coarser than ingots cast in heated molds.

As a specific example of the above process, the following metal alloy was extensively investigated, utilizing 0.03 of a percent by Weight of the inoculant specified in Table I. The inoculant particle size was No. 10 mesh and finer. The inoculant was contained in a copper tube 3" long, /2" in diameter and with a wall thickness of approximately 0.050". A ferrous stainless alloy containing approximately 70% iron, 25% chromium and 5% aluminum was furnace inoculated at a temperature of 2,885 The inoculation was performed by plunging in the above noted copper tube containing the inoculant. The alloy was then poured from the furnace at a temperature of 2,840 P. into molds heated to about 750 F. Both permanent molds and sand molds were utilized. Round ingots approximately 5 in diameter were poured. The ingots were approximately 2 feet in length. The ingots Were allowed to cool and cross sections were cut at various locations along the length of each ingot. The cross sections were polished and etched using an etch formula of the following composition: one part nitric acid, one part hydrochloric acid and one and one-half parts hydrogen peroxide. The cross sections were compared with ingot cross sections prepared in exactly the same manner omitting the inoculation step. In both the permanent mold and sand cast ingot cross sections of the inoculated alloy a uniform fine grain equiaxed macro-structure was present. In the uninoculated cross section the typical columnar grain structure was present with the columnar grains extending from the surface of the ingot to substantially the center of the ingot. The average grain size was reduced by a factor of about 25 for the permanent mold ingot and approximately 50 for the sand mold ingot. A lengthwise section of each ingot showed substantially uniform fine grain structure throughout the length of the inoculated ingots except for the portion near the top Where the final solidification takes place. This portion is normally cut off a cast ingot because of porosity due to both trapped gasses and its use as a riser. The uninoculated ingots showed columnar growth for substantially the entire length of the ingot and a very course grain structure down the center of the ingot with the columnar grains extending almost to the center throughout most of the length of the ingot with the exception of the porous riser portion.

A copper base alloy containing substantially 60% copper, 20% nickel and 20% iron was inoculated with 0.03% by weight of the same inoculant noted in Table I. The inoculation was in the furnace at a temperature of 2,595 F. and the alloy was poured into ingots 2" x 2" in cross section and 48" long at a temperature of 2,550 F. The molds were heated to above 400 F. prior to pouring. The ingots were allowed to air cool and in a similar manner to that above, sections of both inoculated and uninoculated ingots were prepared. The sections were polished and etched utilizing a formula containing 2 parts nitric acid, 2 parts glacial acetic acid and one part acetone. Photomicrographs of the crosssection indicated a fine grain equiaxed micro-structure uniform over the cross sections for the inoculated ingots. The uninoculated ingots showed the common dendritic grain structure for copper base, slowly cooled alloys.

The inoculant required in the above examples constitutes only a small percentage of the melt. More importantly the percentage of high melting point metals in the inoculant constitutes only a small percentage of the inoculant and an extremely small percentage of the melt. Chromium, titanium, zirconium and rhodium act as seeding materials under the theory noted above due to higher melting points than the inoculation temperature for the ferrous alloy; however, the rhodium is most effective. The chromium, titanium and zirconium will tend to partially alloy with the melt if given suflicient time. The iron and nickel may also be considered seeding materials in the copper alloy; however, the inoculation temperature is only slightly below the nickel melting point and therefore the effectiveness of the nickel as a seeding material is questionable. Excessive superheat will also promote alloying of the seeding materials by melting the seeding materials and extending the cooling time between inoculation and pouring. The very small percentages of expensive high melting point materials such as rhodium allow the new process to be used economically. Tungsten and other high melting point materials are also effective and economical in use.

For production purposes the most economical method to heat the permanent molds is to leave the prior ingots in the molds until about mins. before the next pour. The permanent molds will therefore be above 400 F. if the prior ingots are removed when still a dull red in color. Of course the particular molds and environmental surroundings will determine the rate of cooling of a mold and therefore different timing may be required depending upon the production line used.

It is most important in the use of this process that the inoculation take place just prior to pouring. The temperature drop between the inoculation temperature and the pouring temperatures noted above occurs usually during the time necessary for positioning the furnace to pour into the ingots. In most cases a matter of a few minutes. Again this will depend to some extent upon the size of the pour.

The proportions of the various metallic constituents given in Table I for a broad spectrum inoculant are satisfactory for most commercially used metals which have melting points substantially near those of the examples above. In the case of metals having melting points substantially nearer to the lowest melting point constituents or the highest melting point constituents, the proportions of the constituents in the inoculant may be adjusted as an alternative to increasing the percentage of inoculant added. This is because fewer constituents are available for freezing point depression in the low melting point metals and fewer constituents are available for seeding in the high melting point metals. Other constituents may be added or substituted in the inoculant so long as the function requirements for seeding and freezing point depression noted above are met. The inoculant may also contain metallic compounds such as metal oxides and chlorides that meet the requirements as to melting points noted above. The compounds however are not as effective when used for a broad spectrum inoculant unless the low melting point compounds react with the metal to be cast so as to free the metal in the compound for action as a rejected solute when the grains are forming.

What is claimed is:

1. A process for casting a wide variety of ferrous and non-ferrous metals with a fine evenly distributed grain structure including the steps of preparing a casting metal at a super heat temperature slightly above the pouring temperature of said casting metal, inoculating said casting metal with an inoculant suitable for and effective upon said wide variety of ferrous and non-ferrous metals including said casting metal and containing more than two metals wherein at least one metal has a melting point substantially below the solidus temperature of said casting metal, said relatively low melting point metal having a lower solubility in solid solution with said casting metal than in liquid solution, and at least one metal has a melting point substantially above the liquidus temperature of said casting metal, said relatively high melting point metal having low solubility in said casting metal at said super heat temperature, immediately pouring said casting metal at the pouring temperature into a mold and allowing said casting metal to cool below the solidus temperature of said casting metal, said mold preheated substantially above ambient temperature to provide a relatively low temperature gradient in said casting metal during freezing thereby promoting said fine grain structure, said above steps subsequent to all conventional alloying, degassing and dexodiation steps.

2. The process of claim 1 wherein the mold is preheated to a temperature above 400 F.

3. The process of claim 1 wherein the super heat temperature lies between the pouring temperature and 200 F. above the pouring temperature.

4. The process of claim 1 wherein the inoculant is added to the casting metal while the casting metal is in a melting furnace.

5. The process of claim 1 wherein the inoculant is added to the casting metal while the casting metal is in a ladle prior to pouring.

6. The process of claim 1 wherein a permanent mold is utilized.

7. The process of claim 1 wherein a sand mold is utilized.

8. The process of claim 1 wherein the inoculant constitutes less than 0.1% by weight of the casting metal.

9. The process of claim 1 wherein the inoculant constitutes less than 1% by weight of the casting metal.

10. The process of claim 1 wherein said inoculant contains at least one metallic compound.

11. A process applicable for casting a wire variety of ferrous and non-ferrous metals with a fine evenly distributed grain structure including the steps of preparing a casting metal at a super heat temperature slightly above the pouring temperature of said casting metal, inoculating said casting metal with a broad spectrum inoculant suitable for and effective upon said wide variety of ferrous and non-ferrous metals including said casting metal and containing a plurality of metals having melting points substantially below the solidus temperature of said casting metal, at least one of said relatively low melting point metals having a lower solubility in solid solution with said casting metal than in liquid solution, and a plurality of metal having melting oints substantially above the liquidus temperature of said casting metal, at least one of said relatively high melting point metals having low solubility in said casting metal at said superheat temperature, immediately pouring said casting metal at the pouring temperature into a mold and allowing said casting metal to cool below the solidus temperature of said casting metal, said mold preheated substantially above ambient temperature to provide a relatively low temperature gradient in said casting metal during freezing thereby promoting said fine grain structure, said above steps subsequent to all conventional, alloying, degassing and deoxidation steps.

12. The process of claim 11 wherein the mold is preheated to a temperature between 400 F. and the solidus temperature of the casting alloy.

13. The process of claim 11 wherein said super heat temperature lies between the pouring temperature and 200 F. above the pouring temperature of said casting alloy.

14. The process of claim 11 wherein said inoculant contains at least one metallic compound.

15. The process of claim 11 wherein said inoculant constitutes less than 0.1% by Weight of the casting metal.

16. The process of claim 11 wherein said casting metal is a ferrous alloy containing over 50% iron.

17. The process of claim 16 wherein said casting metal is a stainless ferrous alloy containing substantial amounts of chromium and aluminum.

18. The process of claim 17 wherein the alloy contains approximately 70% iron, 25% chromium and 5% aluminum.

19. The process of claim 16 wherein the broad spectrum inoculant constitutes less than 0.1% by weight of the casting metal.

20. The process of claim 11 wherein said casting metal is a copper base alloy.

21. The process of claim 20 wherein said casting metal contains substantial percentages of nickel and iron.

22. The process of claim 21 wherein the alloy contains approximately 60% copper, 20% nickel and 20% iron.

23. The process of claim 20 wherein the broad spectrum inoculant constitutes less than 0.1% by weight of the casting metal.

24. A process for casting a wide variety of ferrous and non-ferrous metals with a fine evenly distributed grain structure including the steps of preparing a casting metal at a superheat temperatureslightly above the pouring temperature of said casting metal, inoculating said casting metal with a broad spectrum inoculant consisting essentially of substantial amounts of copper, iron and zinc with minor amounts of manganese, chromium, titanium, nickel, zirconium, rhodium and lead, pouring said casting metal at the pouring temperature into a mold and allowing said casting metal to cool below the solidus temperature of said casting metal.

25. The process of claim 24 wherein said mold is preheated to a temperature above 400 F.

26. The process of claim 24 wherein the inoculant substantially comprises the percentages of constituents denoted in Table I.

27. The process of claim 26 applied to a casting metal consisting essentially of 70% iron, 25% chromium and aluminum.

28. The process of claim 26 applied to a casting metal 8 consisting essentially of copper, 20% nickel and 20% iron.

References Cited UNITED STATES PATENTS 2,380,566 7/1945 Wyllie 75-153 X 3,383,202 5/1968 Lynch 75-53 X 2,563,056 8/1951 Miller 75-53 3,375,105 3/1968 Lynch 75-129 2,810,640 10/1957 Bolkcom et al 75-129 X 2,659,669 11/1953 Miller 75-129 3,336,118 8/1967 Newitt 75-130 R 2,955,933 10/1960 Freeman 75-130 R 2,826,497 3/1958 Gagnebin et al 75-130 R 3,459,540 8/1969 Tisdale et a1. 75-53 X 2,778,079 1/1957 Carney et a1 164-57 3,259,948 7/1966 Feagin 164-24 3,552,479 1/1971 Hockin 1 64-65 OTHER REFERENCES James L. Walker, et al.: Refining Grain Structure by Inoculation, General Electric Review, July 1958, pp. 26 and 27.

DEWAYNE, RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner U.S. Cl. X.R 164-51, 65

UNITED STATES PATENT OFFICE 569 CERTIFICATE OF CORRECTION Patent No. 3,744, 997 Dated July 10, I973 Inventor(s) Lawrence A- D imen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 11, line 1, correct "wire" to "Wide".

Signed and sealed this 20th day of November 1973.

(SEAL) fittest:

EDWARD M.FLETGHER,JR. RENE D TEGTT IEYER Attesting Officer Acting Commissioner of Patents 

